Pathological Anatomy.—­When dealing with their classification we gave in outline the main pathological changes to be met with in corns.  It now only remains to give the same matter in slightly greater detail.

In dry corn the changes we meet with are those accompanying blood extravasation.  From excessive compression of the parts, or from the effects of direct injury, a portion of the sensitive sole has become lacerated.  The escaping blood stains the surrounding soft tissues after the manner of blood extravasation elsewhere.  If the escape of blood is sufficiently large, the horn fibres in the immediate vicinity also are stained.  It is this stain in the horn that is the direct evidence of the injury, and is itself popularly known as the corn.  It may vary in size from quite a small spot to a broad patch as large as half a crown, while its colour may be a uniform red, or a mottled red and white.  The microscopic changes in this connection are illustrated in Fig. 99.

[Illustration:  FIG. 99.—­HORIZONTAL SECTION OF A CORN.  The section cut at about the base of the papillae of the sensitive sole. a, papillae, with horn-cells surrounding them; b, interpapillary or intertubular horn; c, hollow spaces in the intertubular material filled with blood; d, a papilla and its surrounding horn-cells filled with blood.]

Ordinarily, this ecchymosis of the horny sole is due to injury of the sensitive sole immediately beneath it.  It may, however, proceed from injury to the vessels of the laminae either of the bars or of the wall.  In this case the ecchymosis of the horny sole may be explained by the fact that the escaped blood tends to gravitate to that position.

When the corn is of long standing, or is due to repeated injuries on the same spot, the horn adjacent to the lesion becomes hard and dry, and often abnormally brittle, simply on account of the inflammatory changes thus kept in continuation.  This is often seen when attempts are made to pare out the corn with the knife.

Should the injury be seated in the sensitive laminae, then the brittle nature of the horn secreted by the injured tissues makes itself apparent by the appearance of cracks in the wall of the quarter.  Why this should occur will be readily understood by a reference to Fig. 100.

[Illustration:  FIG. 100.—­INNER SURFACE OF THE WALL OF THE QUARTER, SHOWING CHANGES IN THE HORNY LAMINAE BROUGHT ABOUT BY CHRONIC CORN.]

It will here be seen that the injury to the keratogenous membrane has led to great interference with the secretion of horn from the sensitive laminae.  As a result, the regularly leaf-like arrangement of the horny laminae has been largely broken up.  Certain of the laminae are altogether wanting, while others are broken in their length and rendered incomplete.  With this condition there is always more or less contraction of the quarter.